1. Field of the Invention
The present invention relates generally to a circuit for and method of processing an audio frequency signal and more particularly relates to hearing aid signal processing.
2. Description of the Prior Art
It is well known in the art to utilize electronic devices to assist the hearing impaired. The earliest such instruments consisted of a microphone coupled to an electronic amplifier which was in turn coupled to an earphone. Quite apart from the technical difficulties experienced, these early hearing aids were sufficiently large and intrusive that the hearing impaired could be readily identified providing a degree of self-consciousness.
The coming of electronic miniaturization and sub-miniaturization permitted the manufacture of hearing aid systems which are totally inserted in the outer auditory canal during use. The resulting systems provide substantially greater hearing assistance along with a much more pleasing (and almost unnoticeable) aesthetic appearance. A modern, totally in-the-ear device has a microphone acoustically coupled to the ambient with all of the electronics packaged in a form factor which is accommodated by the outer ear of the patient. A transducer is electronically coupled to the output stage of the hearing aid circuit and acoustically coupled to the distal portion of the outer auditory canal.
U.S. Pat. No. 4,689,818, issued to Ammitzboll on Aug. 25, 1987, purports to describe the circuitry and operation of the Siemens Custom In-The-Ear Hearing Aid 007, sold by Siemens Hearing Instruments, Inc. This is a typical example of a totally in-the-ear device.
A key problem in the miniaturization process is reducing the size of the battery. Whereas substantial progress has been made in battery development, much credit is also appropriately given to designers of low power consumption electronic circuitry. Current state of the art instruments utilize class D output stages which are particularly helpful in reducing overall power consumption. However, as is known to those of skill in the art, the class D output stage tends to have a frequency response curve whose peak gain frequency is not easily modified to accommodate differences in patient pathologies. Yet, abnormalities in middle ear functioning are known to shift the peak in the unaided ear canal resonance to a lower frequency.